How can inverter be more efficient?

Hi,
I fail to see how inverter heat-pumps can be more efficient than fixed-
speed ones.
I guess this boils down to another question: a heat-pump probably has
a speed where its efficiency is maximum. Hopefully this is where fixed-
speed heat-pumps are set to operate. Is that right?
If so, when the inverter varies the speed, it moves away from the
optimum, and efficiency goes down.
It would seem to me, from a pure COP view point, and if my assumptions
are correct, that a fixed-speed heat-pump would have the maximum
efficiency. Now, comfort is also important, and off-on maximum heating
a not as nice as sustained, adapted, heating.
But this is not what is advertised. All vendors claim that COP is
better with an inverter, even though i assume there are lossses in the
current-inverting circuitry. How can that be? The only way I can see
that this would be true is if the COP increases as compressor speed
decreases, but I was never able to find any information about that.
And also -this is presumably linked- why do smaller heap-pumps have a
better COP than larger capacity ones?
Does anyone know the answer to this mystery?
Thanks you for your lights, Chris

I'd love to hear from the experts as well, but until they
get back to us here's my guess:
I believe a heat pump (whether operating as an a/c or as a heater...)
will have better efficiency when there's a lower temperature
difference across it. In other words, if you've got an inlet
temperature of 80 and an outlet of 70, the system is working
more effeciently than if it's pumping out 50 degree air.
Now it'll have to work 3X as long (per cycle time) with
the lower difference, but it'll be more than 3X as
efficient, so you'll come out ahead.
A "regular", or fixed speed, compressor (and the
rest of the system), will always be trying to give
that 30 or so degree temperrature change to the air
flowing through. (It'll cycle on and off, of course,
to match the thermostat setting).
So that inverter system will just gently give you
that five degree drop if that's all that's called for,
and will be much more efficient for it.
As an analogy, think of the MPG you get in a car
running at 90 mph versus 40 mph... While the rolling
resistance increases on a pretty linear slope,
the air resistance is exponential.

There you've got me... In fact, I'm in shock over
the latest offering from Mitsubishi, where they
have a new 9,000 BTU mini split (part of their
"Mr. Slim" line) with a claimed...
23 SEER.

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Knowledge may be power, but communications is the key

The optimum operating point is not fixed - it's dependant on the
conditions at which the heat pump is operating at that particular
time.
The advantage of the inverter is that it allows the controller to
measure those operating conditions and then match the speed of the
compressor and fans to that load.

When a system cycles on and off the average EER is not the rated EER.
So a unit that may have a lower EER and does less cycling can actually
have a higher average EER. Than the unit that cycles more often.
Andy

When a system cycles on and off the average EER is not the rated EER.
So a unit that may have a lower EER and does less cycling can actually
have a higher average EER. Than the unit that cycles more often.
Andy

The inverter based systems start off at a dis-advantage in that the
"electronics" will consume a few percent of the power.
BUT they have several advantages:
1) They maintain efficiency when operating motors at lower speeds. Your
run of the mill 3 speel HVAC motor, for example, can have less than 50%
efficiency. An "inverter" based fan motor can be 90+% efficient at all but
the very lowest speed.
2) Induction motors have some compromises to enable them to start and
these lower the efficiency at rated speed. (The rotor internal resistance
is higher.) The "inverter" motor starts at low frequency and low voltage.
It's just a matter of time before the inverter technology complely subplants
"tranditional" replacement motors for the air handlers in heat pumps and
A/C. The distributors will only have to stock a literal handful of sizes
and the user will "dial up" the speed he wants. The motors will run
equally well at 120 or 240 volts.
Likewise, "inverter" designs can eliminate the need for power hogging
contactors and relays. That will open the door for "smarter" thermostats
that would only require a two wire connection to the air handler. Such
thermostats with semi-independent control of fan speed and compressor speed
can control humidity or optimize efficiency as the customer desires.

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